It was first shown in 1972 that by superimposing linear field gradients to the static magnetic field of a nuclear magnetic resonance (NMR) experiment it is possible to obtain three-dimensional images of an object (Lauterbur, P. C., Nature 1973, 190). The new technique became known as magnetic resonance imaging (MRI). MRI has developed impressively, becoming one of the most powerful tools “to look inside matter” (Aime, S. B. et al., E. Acc. Chem. Res. 1999, 32, 941). As X-ray imaging did in the beginning of the 20th century, magnetic resonance imaging (MRI) has revolutionized modern diagnostic medicine (Caravan, P. E. et al., Chem. Rev. 1999, 99, 2293). Whereas conventional X-rays show skeletal structure, MRI enables the acquisition of high resolution, three-dimensional images of the distribution of water in vivo. This powerful diagnostic tool is invaluable in the detection of a wide variety of physiological abnormalities including tumors, lesions, and thromboses. Additionally, recent advances in dynamic MRI open up the exciting possibility of real-time imaging of biochemical activity (“A New Generation of In Vivo Diagnostics,” MetaProbe, 2000).
The medical utility of MRI is enhanced through the administration of contrast agents prior to the scan, which alters the relaxation times of protons in the vicinity of the agent, increasing the degree of contrast between healthy and diseased tissue. The use of contrast agents is increasingly popular in medical protocols, with some 30-35% of MRI scans now acquired with the aid of a contrast agent (Caravan, P. E. et al., Chem. Rev. 1999, 99, 2293; Aime, S. B. et al., E. Acc. Chem. Res. 1999, 32, 941). Consequently, contrast agents now represent a very large market, with sales of over $250 million in 2000 (“RC-211, Contrast Agents for Medical Imaging,” Business Communications Company, Inc., 1999).
Several new contrast agents are currently under development, which are designed to be more site-specific, facilitating, for example, detailed images of cardiovascular features (Lauffer, R. B., Magn. Reson. Med. 1991, 22, 339). Additionally, recent reports have demonstrated that contrast agents can detect the presence of enzymes and metal cations (Moats, R. A. F. et al., Angew Chem., Int. Ed. Engl 1997, 36, 726; Li, W. F. et al., J. Am. Chem. Soc. 1999, 121, 1413).
At present, clinically accepted contrast agents are based upon a gadolinium complex of a poly(aminocarboxylate) ligand, e.g., the gadolinium chelates of DTPA, DOTA, DO3A and DTPA-BMA (FIG. 1). The agents are extracellular agents that distribute non-specifically throughout the plasma and interstitial space of the body. A typical use of such agents is in the detection of tumors in the brain.
The image enhancing capability of available agents is far lower than the optimal values predicted by theory (Aime, S. B. et al., Coord. Chem. Rev., 321: 185-6 (1999)). The relatively low image enhancing properties of current contrast agents requires injection of gram quantities in order to obtain satisfactory contrast in the resulting image. With such large doses required for reasonable image enhancement, present contrast agents are limited to targeting sites where they can be expected to accumulate in high concentrations. To accomplish greater resolution with lower dose and to enable a variety of target-selective imaging (such as hepatobiliary features), there is a need for contrast agents of increased image enhancement capacity and corresponding enhanced water proton relaxivity. Moreover, a useful complex must be highly water soluble, resistant to in vivo dissociation of the metal ion from the chelate and of acceptably low toxicity. A promising new class metal-binding ligands are based upon the heterocyclic pyridinone and pyrimidinone nuclei.
Distinct in both structure and properties from poly(aminocarboxylate) chelate-based MRI contrast agents is a class of compounds that include one or more hydroxypyridinone or hydroxypyrimidinone subunit. Both homopodal and heteropodal chelating agents incorporating a hydroxypyridinone or hydroxypyrimidinone moiety are known in the art. Although many of the reported compounds exhibit the desirable water exchange kinetics and complex stability characteristic of this class of compounds, the reported water solubilities of the Gd(III) complexes are generally insufficient to allow the complexes to be considered as candidate MRI contrast enhancing agents.
For example, Xu et al. (J. Am. Chem. Soc., 117: 7245-7246 (1995) reported the synthesis and characterization of Gd(III) TREN-Me-3,2-HOPO (tris((3-hydroxy-1-methyl-2-oxo-1,2-didehydropyridine-4-carboxamido)ethyl)amine)). The solubility of the disclosed complex in water is only about 0.1 mM, making it less than ideal as a MRI contrast enhancing pharmaceutical.
Furthermore, Cohen et al. (Inorg. Chem., 39: 5747-5746 (2000)) prepared a series of mixed ligand systems that are based on the TREN-Me-3,2-HOPO platform. The ligands include two HOPO chelators and a non-HOPO chelator. The ligands set forth in Cohen et al. incorporate salicylamide, 2-hydroxyisophthalamide, 2,3-dihydroxyterephthalamide and bis(acetate) as the non-HOPO chelators. The Gd(III) complexes of the ligands according to the disclosed motif were of moderate water solubility (approx. 1-3 mM).
Hajela et al. (J. Am. Chem. Soc. 122: 11228-11229 (2000)) prepared a homopodal Me-3,2-HOPO chelate based on a functionalized TREN backbone. The functionalized TREN backbone was a homochiral tris(2-hydroxymethyl)-TREN-Me-3,2-HOPO. The Gd(III) complex of the ligand has a water solubility of approximately 15 mM.
HOPO ligands in which the endocyclic nitrogen of the pyridinone moiety is functionalized are known. For example, the ligand TREN-MOE-3,2-HOPO (tris(3-hydroxy-1-methoxyethyl)-2-oxo-1,2-didehydropyridine-4-carboxamido)ethyl)amine) and its Gd(III) complex was prepared and characterized by Johnson et al. (Inorg. Chem. 39: 2652-2660 (2000)). The complex was reported to have a water solubility of about 1 mM.
In addition to those references discussed above, U.S. Pat. No. 5,049,280 discloses homopodal chelating agents based on the 2,3-dihydroxyterephthalamide moiety. The '280 patent does not disclose or suggest combining the disclosed moiety with a HOPO or HOPY subunit to form a heteropodal chelating agent. U.S. Pat. No. 4,698,431 discloses chelating agents having 1-hydroxy-2-pyridinone subunits. The complexes are disclosed to exhibit a high affinity for iron ions and actinides such as Pu(IV). Neither the '280 nor the '431 patent suggest the use of the novel chelates to complex Gd(III).
U.S. Pat. No. 5,892,029 and 5,624,901 set forth a class of homo- and heteropodal chelate systems having at least one 3,2-HOPO subunit within their structure. Neither the '029 nor the '901 patent suggest that the disclosed ligands are of use in forming a highly water soluble Gd(III) chelate.
Other related art includes U.S. Pat. No. 4,666,927, which discloses a number of chelating agents having 1,2-HOPO, 3,2-HOPO, or 3,4-HOPO moieties incorporated within their structures that are linked through a number of possible combinations of linking groups, including —CONH— groups. However, U.S. Pat. No. 4,666,927 teaches against a HOPO moiety having a substitution ortho to the hydroxy or oxo group of the HOPO ring. U.S. Pat. No. 6,221,476 discloses polyhydroxypridinone ligands that are attached to a membrane support. The compositions are useful for removing metal ions from solutions. Zbinden (U.S. Pat. No. 5,688,815) sets forth a class of 3-hydroxypyridin-4-ones, which are effective chelators of iron and useful to treat iron overload. Neither the '927 nor the '476 teach one of skill in the art how to prepare a highly water soluble Gd(III) chelate that is of use as a contrast enhancing pharmaceutical.
As discussed above, Gd(III) chelates of hydroxypyrimidinone and hydroxypyridinone ligands have a number of properties that make them superior MRI contrast agents relative to the widely used Gd(III)-poly(aminocarboxylate) agents. The development of the new contrast media based on the pyrimidinone and pyridinone ring systems has, however, been hampered by the inadequate water solubility of such agents. A new generation of Gd(III) complexes based upon the heterocyclic ring systems would represent a significant advance in the field of MRI contrast enhancement. Quite surprisingly, the present invention provides such complexes.